1. Field of the Invention
This invention relates to a keyboard switch for use in a data entry system for a computer, a typewriter, a word processor and the like, and more particularly to an improvement in an actuator or the keyboard switch which acts upon a membrane switch by vertical movement of a key stem of the switch.
2. Description of the Prior Art
FIGS. 3A and 3B illustrate a conventional keyboard switch. The keyboard switch includes a frame 1 which serves as a housing and a stationary side of the switch. The frame 1 is formed at an upper surface thereof with a circular aperture 1a through which a cylindrical key stem is inserted so that it may move vertically. A key top 3 is mounted on an upper end of the key stem 2, while a pressing member 4 is fixed to a lower end of the key stem Z. The pressing member 4, as shown in FIGS. 4(a) and 4(b), is formed into a substantially annular shape and includes a rear section 4a which is integral with a side 2a of the key stem 2 to be connected in a cantilever fashion by means of a neck portion 5. The pressing member 4 also includes a front section 4b having a pressing projection 6 on the rear surface thereof. The pressing member 4 is resilient in the vertical direction. The frame 1 includes a cavity in which a sheet-like contact member (hereinafter referred to as "membrane switch") 7 is arranged on a substrate 8 of the frame 1. The membrane switch 7 is formed of two films 9 and 10 having an upper contact 9a and a lower contact 10a, respectively, and arranged in a manner to be opposite to each other with a spacer 11 therebetween. The contacts 9a and 10a are positioned right under the pressing projection 6 provided at the pressing member 4 of the key stem 2. Between the lower end of the key stem 2 and the sheet 9, a return spring 12 is arranged to bias the key stem 2 upwardly.
In the conventional keyboard switch described above, when the key top 3 is depressed with a finger, the key stem 2 slides into the frame 1 while compressing the return spring 12 to cause both contacts 9a and 10a of the membrane switch 7 to contact with each other by means of the pressing projection 6, thereby to close the switch.
When assembling the keyboard switch, the key stem 2 is inserted through the aperture 1a from the lower side of the frame 1 and the return spring 12 is put on the lower end of the key stem 2 which projects in the frame 1. The return spring 12 is then held by the substrate 8 on which the membrane switch 7 is fixed, while aligning the substrate 8 with the frame 1. In this condition, the return spring 12 is held at a somewhat compressed state, because it is to impart returning force to the key stem 2. The return spring 12 extends beyond the lower surface of the frame before the membrane switch 7 is put on the frame 1, and is liable to deform its shape during the assembling, because it is made independent from the key stem 2, which makes it difficult to fix the membrane switch 7 and substrate 8 with respect to the frame 1 while aligning them with the frame 1. Furthermore, the conventional keyboard switch not only has the drawback associated with the assembling of the switch resulted from the return spring 12 and key stem 2 which are made independent from each other, but also it is high in manufacturing costs, because there are a number of parts to be assembled.
In addition, the conventional keyboard switch causes the key stem 2 to strike at its lower end directly against the membrane switch 7 on the stationary side of the frame 1 when the key stem 2 is depressed. The repeating operation of such a keyboard switch sometimes damages fingers of a keyboard operator by shock due to the strike, and also generates noise. The noise is generated due to strike of the key stem 2 against the frame as well when the key stem 2 is returned to the original position.